Polyurethane coated safety glass

ABSTRACT

A scratch-resistant polyurethane coating for safety windows is made by curing a sheet of a liquid composition comprising (1) a polyglycol ether resulting from the condensation of propylene oxide with trimethylolpropane and containing 10.5%-12.0% free hydroxyls and (2) a biuret of 1,6-hexamethylene-diisocyanate, having 21-22% isocyanate groups, the weight ratio of said biuret to said polyglycol ether being about 0.9-1.1.

.Iadd.The application is a reissue continuation of application Ser. No.06/456,251, filed Jan. 6, 1983, now abandoned, which is a reissuecontinuation of application Ser. No. 06/417,847 filed Sept. 14, 1982 nowabandoned, which is a reissue continuation of application Ser. No.05/847,321 filed Oct. 31, 1977, now abandoned, which is a reissue ofU.S. Pat. 3,979,548 issued Sept. 7, 1976 (application Ser. No. 523,016filed Nov. 12, 1974). .Iaddend.

BACKGROUND OF THE INVENTION

A prime objective of this invention is to provide a polyurethane coatingwhich is scratch-resistant, even when it is manufactured undertemperature conditions of 90° C. or higher.

In the copending and commonly assigned application for U.S. patent Ser.No. 369,087 filed June 11, 1973, there is described the use ofscratch-resistant plastic coatings on the interior surface of safety.[.glass.]. .Iadd.glazing .Iaddend.for automobiles and the like, and inparticular the use of weakly cross-linked aliphatic polyurethanes toform such coatings.

According to this prior copending application such coatings should havethe following properties:

    ______________________________________                                        Modulus of elasticity about                                                                           1,000 kg/cm.sup.2                                     Mean modulus of deformation                                                                           200 kg/cm.sup.2                                       Elongation at break     100%                                                  Breaking strength       100 kg/cm.sup.2                                       Plastic deformation, expressed as                                                                     2%                                                    percentage of total elongation at break                                       ______________________________________                                    

Such polyurethane coatings are made by curing liquid mixtures comprisingtwo constituents, namely (1) an aliphatic polyether having free (i.e.reactive) hydroxyl groups and (2) an aliphatic polyisocyanate havingreactive isocyanate groups. The liquid mixture is cast into a sheetwhich is then cured, whereby hydroxyl groups of the polyether moleculesreact with isocyanate groups of the aliphatic isocyanate molecules toform a three-dimensional cross-linked polyurethane structure.

In German published application No. 2,058,504 there is described anexemplary mode of preparing polyurethanes of this type in whichcomponent (1) is a polyglycol ether resulting from the condensation ofpropylene oxide with trimethylolpropane and in which component (2) is abiuret of 1,6-hexamethylenediisocyanate. In example III of this Germanapplication a coating is prepared by curing a mixture of 128 parts byweight of such a biuret together with 100 parts by weight of such apolyglycol ether. Since this application is silent as to the relativepercentages of reactive hydroxyls and isocyanates respectively in thetwo components it is not clearly defined how close to the stoichiometricthese proportions are.

The general teaching of the prior art is vague in regard to thestoichiometry desirable in forming polyurethanes from polyols andpolyisocyanates. For example, U.S. Pat. No. 3,791,914 states thatordinarily the amount of polyisocyanate utilized is sufficient to reactone equivalent of isocyanate per hydroxyl equivalent of the polyol; butthat this ratio is not critical "except for obtainment of optimumresults, higher or lower amounts being employed without adverseresults".

The present applicants have found that stoichiometrically equivalentproportions are disadvantageous when a high temperature such as about90° C. or higher is used in affixing the polyurethane plastic sheet tothe .[.glass.]. .Iadd.safety glazing.Iaddend.. Such higher temperatureis required particularly when the outer layer of polyurethane is affixed.[.simultaneously with lamination of two or more glass sheets.]..Iadd.to a laminated glazing .Iaddend.interlayered withpolyvinylbutyral. The temperature required for such an assembly isbetween about 120° and 140° C. During such high heat conditions, thephysical properties of the outside polyurethane sheet deteriorate. Inparticular, the scratch-resistance is substantially reduced.

Specifically, present inventors have observed the above describedresults when polyurethane coatings were prepared from mixtures of 100parts by weight of a component (1) being a polyglycol ether resultingfrom the condensation of propylene oxide with trimethylolpropane andhaving 10.5-12% free hydroxyls, together with 128 parts by weight of (2)a biuret of 1,6-hexamethylene-diisocyanate having 21-22% isocyanategroups. The weight ratio W₂ /W₁ of components (2) to component (1),equal to 1.28, corresponds approximately to a stoichiometric equivalencebetween the available isocyanate and hydroxyl groups respectively.

Using a mar test to be described in greater detail below, a coatingprepared from this composition by curing at a temperature below 90° C.,has a normal scratch-resistance equal to about 30 grams. But after atreatment at 90° C. for a half-hour, the scratch-resistance decreases toabout 2 grams. This reduction is so great as to make the productunsuitable for use on the interior surface of a safety windshield in amotor vehicle. The surface layer has become too rigid and too readilyscratched.

SUMMARY OF THE INVENTION

Applicants have found that the aforedescribed deficiencies can beovercome simply by changing the ratio of the same components (1) and (2)so that W₂ /W₁ is approximately equal to 1; i.e. approximately equalweights of the same polyglycol ether and polyisocyanatc are used.

Briefly stated, the present invention comprises curing a sheet of aliquid composition containing (1) a polyglycol ether resulting from thecondensation of propylene oxide with trimethylolpropane and having10.5-12% free hydroxyls and (2) a biuret of 1,6-hexamethylenediisocyanate having 21-22% isocyanate groups, the weight ratio of saidbiuret to said polyglycol ether being about 0.9-1.1.

The present invention relates also to a safety window comprising atleast one glass sheet having bonded thereto a scratch-resistant layer ofthe polyurethane of this invention.

This invention relates also to a safety glass laminate of a plurality ofsolid glass sheets having disposed between adjacent sheets an adherent,transparent interlayer of polyvinylbutyral and having bonded to at leastone outer glass sheet of this laminate an adherent solid transparentlayer of the scratch-resistant polyurethane of this invention.

DETAILED DESCRIPTION

It thus appears surprisingly that a better durability of mar-resistanceon exposure to high temperatures is obtained by departing from astoichiometric ratio, with an excess of hydroxyls over isocyanates.Normally, one would expect such a less rigid and incompletelycross-linked structure to be more easily scratched. Mechanicalcharacteristics in general are also made more durable to heat treatment,as will appear from examination of the annexed drawings, in which:

FIG. 1 shows the variation in breaking strength, expressed in kilopondsper square centimeter (or kilograms of force per square centimeter), ofthe cured polyurethane sheeting as a function of the above definedweight ratio W₂ /W₁ of components (2) to (1). The breaking strength wasmeasured according to German Industrial Standards DIN 53455.

FIG. 2 is a graph of the variation in elongation at break, expressed inpercent of original length, as a function of the same ratio W₂ /W₁. Thestandard tests were likewise made in accordance with DIN 53455.

FIG. 3 is a graph of the variation of tear resistance as a function ofW₂ /W₃. The tear resistance was measured according to German IndustrialStandards DIN 53515 and is again expressed in kiloponds per centimeter.

FIG. 4 is a graph of the variation of scratch-resistance expressed inponds as a function of the same ratio W₂ /W₁. Scratch-resistance wasmeasured with an instrument, Type 413, manufactured by the firm Erichsenat Hemer. This instrument involves a diamond-pointed stylus which can beloaded with various weights and which moves in a circular path over thesurface whose resistance to scratching is to be evaluated. The minimumweight which produces permanent scratches constitutes a measure of theresistance of the surface to scratching.

In these four graphs, the unbroken curves C present the values obtainedon samples not subjected to any temperatures above 90° C., whereas thebroken curves D present the values obtained on corresponding samplessubmitted to higher temperatures, exemplarily to 90° C. for 1 hour. Itis to be emphasized that a temperature around 90° C. is a thresholdbeyond which there is little further change in the respective physicalproperties. Thus, values obtained for samples exposed to temperatures upto 140° C., for example, fit on the same broken lines. Thus, the brokencurves shown are sufficient to show the effect of thermal treatment ingeneral.

If a temperature higher than 90° C. is used, the time required toachieve maximum rigidity is simply shortened, but values of the physicalproperties finally reached are substantially the same.

It will be observed in FIG. 1 that a heat treatment about 90° C. (curveD) increases breaking strength substantially when the ratio W₂ /W₁ isgreater than about 1.1, whereas when W₂ /W₁ has the values of thisinvention, between 0.9 and 1.1, heat treatment has no influence on thebreaking strength and curves C and D coincide.

The data on elongation to break presented graphically in FIG. 2 showthat an optimum value of 100% or more elongation to rupture is obtainedeven after heat treatment, when the value of W₂ /W₁ is in the vicinityof unity. but that there is a substantial deviation of the D-Curve fromthe C-Curve when W₂ /W₁ rises above or falls below the unity value.

The tear resistance curves of FIG. 3 have, as is to be expected, thesame general shape as those of FIG. 1. These curves show that betterinitial tear resistance is obtained as the ratio W₂ /W₁ increases to the1.28 corresponding to stoichiometric equivalence, and beyond; and thatthe exposure to heat actually increases the tear resistance. However,this advantage is obtained to the detriment of the other properties asis apparent from FIGS. 1, 2 and 4. Applicant has determined that thetear resistances obtained when W₂ /W₁ is between 0.9 and 1.1 (and whichare unchanged by heat treatment) are commercially acceptable in theutilization of these layers, particularly when coating safety.[.glass.]. .Iadd.glazing .Iaddend.on the surface facing the interior ofa vehicle.

The curves of FIG. 4 show clearly the advantage obtained by the processof this invention. It is evident that products with W₂ /W₁ substantiallyexceeding unity have significantly diminished scratch-resistance evenwithout high heat treatment (Curve C) and that heat treatment at greaterthan 90° C. substantially decreases this resistance by amounts whichmake the product commercially unacceptable. In particular a ratio of W₁/W₁ =1.28, corresponding to stoichiometric equivalence, is completelyunsatisfactory.

The overall thrust of the information in these figures is that theweight ratio of polyglycol ether to biuret components of the compositionof this invention should be selected between 0.9 and 1.1, preferablyclose to unity. in order to make a laminate of the cured polyurethane on.[.glass.]..Iadd.safety glazing .Iaddend.which can be subjected to hightemperatures. It may be confidently predicted that this region ofcomposition is also advantageous even when the layer is not subjected toa high temperature treatment, inasmuch as it is well known that thedeterioration process which takes place at elevated temperaturesinvolves an acceleration of aging phenomena which take place more slowlyat lower temperatures.

FIGS. 5 and 6 illustrate assemblies in which the scratch-resistantcoating of this invention is bonded to a glass sheet. In FIG. 5 there isexemplarily one glass sheet 1 and one layer 2 of the polyurethanecoating of this invention adhered thereto. In FIG. 6 there isillustrated an assembly of at least two sheets 3 and 4 of glass havingdisposed between them sheets of an adherent transparent interlayer 5 ofpolyvinylbutyral and having bonded to at least one outer glass sheet ofthis laminate an adherent solid transparent layer 6 of thescratch-resistant polyurethane of this invention.

In preparing the polyurethane coating composition from a mixture ofcomponents 1 and 2, it is advantageous to include also an accelerator.Substances which are particularly suitable as accelerators are thedialkyltin carboxylates and stannous carboxylates, including exemplarilydibutyltin di-(2-ethylhexoate), dibutyltin dilaurate, stannous oleateand stannous octoate. Zinc carboxylates such as zinc octoate or zincnaphthenate can also be used. Dibutyltin is preferred.

Since the accelerator can also catalyze the aging of the cured film, theweight proportion of accelerator, based on the polyglycol component 1,should not be greater than about 0.1%. It is preferred that the weightratio of accelerator to component 1 should be between about 0.01 and0.07%. The most suitable value of such ratio is about 0.045-0.055%.

It is also suitable, as known, to add along with the accelerator anantioxidant stabilizer, for example one of the polyalkylphenols havingsterically hindering substituent groups in one or both of the orthopositions. A particularly suitable stabilizer is2,6-di(tert.butyl)-p-cresol. The weight percentage of stabilizer, basedon the polyglycol component 1 can be between 0 and 5%, suitably between1 and 5%, the preferred percentage being about 2-2.5%.

EXAMPLE OF THE INVENTION

To achieve a polyurethane coating composition according to thisinvention, a mixture is made, at a temperature between about 20° and 50°C., as follows: To the mixing vessel there is first added 1000 grams ofthe polyglycol ether resulting from the condensation of propylene oxidewith trimethylolpropane, having a molecular weight about 450 andcontaining 10.5-12% free hydroxyls. To this polyglycolether there isadded 23 grams of 2,6-di(tert.butyl)-p-cresol as stabilizer and 0.5grams of dibutyltin dilaurate as accelerator. When these ingredients arewell-mixed, there is further added 1000 grams of the biuret of1,6-hexamethylene diisocyanate having 21-22% isocyanate groups.

A homogeneous mixture is obtained by stirring carefully to avoid theformation of air-bubbles or blisters.

This liquid mixture is then cast into a film by pouring on to asubstrate or support exemplarily of glass, where it polymerizes or curesto acquire its characteristic film structure. The curing or drying time,which is 24 hours at 20° C., can be reduced to 15 minutes by heating at140° C. If it is desired to remove the layer of cured film from itssupport, the surface of the glass or other support is advantageouslyfirst treated with a release agent such as magnesium stearate.

The cured coating or film prepared in this manner has the followingproperties:

    ______________________________________                                        Breaking strength    100-140 kg/cm.sup.2                                      Elongation at break  100-135%                                                 Tear Resistance      15 kg/cm                                                 Scratch Resistance   25-30 grams                                              ______________________________________                                    

A particular advantage of the liquid composition of this invention isits relatively low and manageable viscosity before curing.

If, contrary to the teaching of the instant invention, the weights W₁and W₂ respectively of components 1 and 2 are used in proportion so thatW₂ /W₁ =1.28 (i.e. 128 parts by weight of component 2 are mixed with 100parts by weight of component 1, it is extremely difficult to operatewithout addition of solvent because the viscosity of this mixture is4,600 centipoises at 20° C. On the other hand when the ratio of W₂ /W₁=1.00 as in this example of the instant invention, the liquidcomposition has a viscosity of only 2,800 centiposies at the sametemperature.

It is of course possible to decrease the working viscosity of prior-artcompositions by adding solvents. However, this gives rise to otherdisadvantages. The time required for drying and curing is prolonged.More elaborate drying equipment is necessary, including installationsfor solvent recovery. Additional expenses are involved both for buildingthe equipment and for operating with the elevated output of heat energyneeded.

Furthermore, in addition to these disadvantages in handling, it isextremely difficult to eliminate solvent residues completely, givingrise to deficiencies in the final curing at elevated temperatures suchas blistering and non-uniformity of adhesion.

By avoiding the necessity of resorting to solvents, the compositions ofthe instant invention are substantially improved over the prior art.

We claim:
 1. A safety window comprising at least one glass sheet having bonded thereto an adherent solid transparent layer of a scratch-resistant polyurethane formed by curing a liquid mixture containing (1) a polyglycol ether resulting from the condensation of propylene oxide with trimethylolpropane and containing about 10.5-12% by weight free hydroxyls together with (2) a biuret of 1,6-hexamethylene diisocyanate having about 21-22% by weight isocyanate groups, the weight of said biuret being between about 0.9 and 1.1 times the weight of said polyglycol ether.
 2. A safety glass laminate of a plurality of solid glass sheets having disposed between adjacent sheets an adherent, transparent solid interlayer of polyvinylbutyral and having bonded to at least one outer glass sheet of this laminate an adherent solid transparent layer of scratch-resistant polyurethane formed by curing a liquid mixture containing (1) a polyglycol ether resulting from the condensation of propylene oxide with trimethylolpropane and containing about 10.5-12.0% by weight free hydroxyls together with (2) a biuret of 1,6-hexamethylene diisocyanate having about 21-22% by weight isocyanate groups, the weight of said biuret being between about 0.9 and 1.1 times the weight of said polyglycol ether. .Iadd.
 3. A two-layered article comprising a layer of a scratch-resistant polyurethane cast onto a casting support, wherein said polyurethane layer is formed by curing a liquid mixture containing (1) a polyglycol ether resulting from the condensation of propylene oxide with trimethylolpropane and containing about 10.5-12% by weight free hydroxyls together with (2) a biuret of 1,6-hexamethylene diisocyanate having about 21-22% by weight isocyanate groups, the weight of said biuret being between about 0.9 and 1.1 times the weight of said polyglycol ether. .Iaddend. .Iadd.4. The two-layered article of claim 3, wherein said support is comprised of glass. .Iaddend. .Iadd.5. The two-layered article of claim 3, wherein said casting support is provided with a release agent between said support and said polyurethane film. .Iaddend. .Iadd.6. The two-layered article of claim 5, wherein said release agent is magnesium stearate. .Iaddend. 